Vaso-occlusive phenomena are responsible for much of the acute morbidity of sickle cell disease, including episodes of pain resulting from bone infarcts, splenic infarction with a secondary increased risk of infection, and a relatively high incidence of ischemic stroke (~10% in the first 2 decades of life). In addition, chronic and cumulative ischemic episodes contribute to long-term morbidity (including avascular necrosis of bone, retinopathy, renal insufficiency, and pulmonary hypertension) and a significantly shortened life span. Vaso-occlusive pain crises often require hospitalization for the administration of parenteral narcotics; the average duration of hospitalization is 4-5 days, and a significant proportion of patients experience multiple crises per year. In most episodes, pain continues to intensify over the first 2-3 days before beginning to abate, suggesting that there is ongoing extension of tissue damage for some time following initiation of the episode; also, many patients will develop additional foci of pain even during the course of hospitalization.